Hi Meike,
Two things - a visa into any of the East African countries will allow you to leave your country and go into another one and return without needing a multiple entry visa - i.e. if you get a single entry Ugandan visa you can leave to Kenya and return to Uganda without needing to get a new visa in Uganda...think that will save you lots of funds.

On the lonely planet website there is a link for travel insurance that so far looks decent...I've more research to do, but there you go.

Don't forget immunizations!

HAVE FUN!